1. Field of the Invention
The present invention relates to a device (hereinafter referred to as xe2x80x9clight-emitting devicexe2x80x9d) having a n element (hereinafter referred to as xe2x80x9clight-emitting elementxe2x80x9d) in which a thin film made of a light-emissive material is interposed between a pair of electrodes (an anode and a cathode). In particular, the invention relates to a light-emitting device having a light-emitting element (hereinafter referred to as xe2x80x9cEL elementxe2x80x9d) using a thin film that is made of a light-emissive material capable of EL (electroluminescence). The organic EL display and the organic light-emitting diode (OLED) are included in the light-emitting device according to the invention.
All light-emissive materials that emit light (phosphorescence and/or fluorescence) through singlet excitation or triplet excitation or both are part of the light-emissive materials that can be used in the invention.
2. Description of the Related Art
Light-emitting devices (hereinafter referred to as xe2x80x9cEL light-emitting devicesxe2x80x9d) having an EL element are now being developed. EL light-emitting devices are classified into the passive matrix type and the active matrix type, and EL light-emitting devices of both types operate on the principle that a current is caused to flow through an EL element and thereby a thin film (light-emitting layer) made of a light-emissive material capable of EL emits light.
FIG. 2 shows the structure of a general EL element. As shown in FIG. 2, an EL element 200 is formed by laying an anode 202, a light-emitting layer 203, and a cathode 204 one on another on an insulator 201. In general, the cathode 204 as an electron supply source is a metal electrode having a small work function and the anode 202 as a hole supply source is an oxide conductive film (typically, an ITO film) that has a large work function and is transparent to visible light. This is because the metal electrode as the cathode 204 is opaque to visible light and hence light (hereinafter referred to as EL light) that is generated in the light-emitting layer cannot be observed unless the anode 202 is transparent to visible light.
The EL light 205 is observed after directly passing through the anode 202 or being reflected by the cathode 204 and then passing through the anode 202. That is, an observer 206 can observe the EL light 205 that originates from pixels where the light-emitting layer 203 emits light and that then passes through the anode 202.
However, there is a problem that incident ambient light (i.e., light outside the light-emitting device) 207 is reflected by the back surface (i.e., the surface adjacent to the light emitting layer) of the cathode 204 in pixels that are not emitting light and the back surface of the cathode 204 acts like a mirror, whereby an external view appears on the observation surface (i.e., the surface on the observer 206 side). One measure against this problem is to attach a circularly polarizing film to the observation surface of an EL light-emitting device. However, a problem still exists that the circularly polarizing film is very expensive and hence its employment increases the manufacturing cost.
The present invention has been made in view of the above problems in the art, and an object of the invention is therefore to prevent an EL light-emitting device from acting like a mirror surface without using a circularly polarizing film and thereby provide an inexpensive EL light-emitting device through reduction of its manufacturing cost.
Another object of the invention is to provide an inexpensive electric apparatus having such an EL light-emitting device as a display unit.
In a light-emitting device according to the invention, it is characterized that both of a pair of electrodes (an anode and a cathode) that constitute a light-emitting element (typically, an EL element) are formed from the conductive film that is transparent or semitransparent to visible light, and that a light shield film is provided adjacent to the light-emitting element or provided above or below the light-emitting element with an insulating film or a conductive film interposed in between. That is, it is characterized that a light-emitting layer and the light shield film are provided with one of the pair of electrodes interposed in between, or that the light shield film is provided on the anode side or the cathode side of the light-emitting element.
In this specification, the term xe2x80x9ctransparent to visible lightxe2x80x9d means that the transmittance for visible light is 80-100% and the term xe2x80x9csemitransparent to visible lightxe2x80x9d means that the transmittance for visible light is 50-80%. Naturally, the transmittance depends on the film thickness. The film thickness may be designed properly so that the transmittance falls within the above range.
FIG. 1A shows the character of a light-emitting device according to the invention. As shown in FIG. 1A, a light shield film 104 is provided adjacent to an EL element 100 that consists of an anode 101, an EL layer 102, and a cathode 103. The anode 101 and the cathode 103 are transparent or semitransparent to visible light. To this end, it is preferable that the anode 101 be an oxide conductive film having a work function of 4.5-5.5.
The cathode 103 needs to be a conductive film (typically, a metal film containing a group-1 or group-2 element in the periodic table) having a work function of 2.0-3.5. Since in many cases such a metal film is opaque to visible light, it is preferable to employ a structure shown in FIG. 1B, which is an enlarged view of the EL element 100. As shown in FIG. 1B, the cathode 103 consists of a semitransparent electrode 103a and a transparent electrode 103b. 
The semitransparent electrode 103a is a metal film containing a group-1 or group-2 element in the periodic table. Being as thin as 5-70 nm (preferably, 10-30 nm), the metal film is semitransparent to visible light. The transparent electrode 103b is an oxide conductive film that is transparent to visible light.
The EL layer 102 may have a known structure. That is, the EL layer 102 may be an undoped light-emitting layer or a light-emitting layer containing a dopant (e.g., an organic material that emits light through triplet excitation).
A lamination structure may be formed in which a carrier (electron or hole) injection layer, a carrier transport layer, or a carrier stop layer is laid on a light-emitting layer.
Although in FIG. 1A the light shield film 104 is provided adjacent to the anode 101, it may be provided adjacent to the cathode 103. An insulating film or a conductive film may be provided between the light shield film 104 and the anode 101 (or between the light shield film 104 and the cathode 103).
In the invention, the light shield film may be a thin film that is made of a material having a large absorption coefficient for visible light. Typical examples of the light shield film are an insulating film (preferably, a resin film) dispersed with metal particles or carbon particles, a metal film (preferably, a titanium film, a titanium nitride film, a chromium film, a molybdenum film, a tungsten film, a tantalum film, or a tantalum nitride film) having a small reflectance value, and a semiconductor film.
With the structure of FIG. 1A, EL light 105 can pass through the cathode 103 and hence is observed directly by an observer 106. Most of ambient light 107 reaching the light shield film 104 is absorbed by the light shield film 104 and hence reflection light is weak enough to be unproblematic. That is, the reflection light does not reach the observer 106 and hence the problem that an external view appears on the observation surface can be solved.
Capable of solving the problem in the prior art without the need for using an expensive circularly polarizing film, the invention makes it possible to provide inexpensive light-emitting devices. Further, inexpensive electric apparatuses can be provided by using a light-emitting device according to the invention as a display unit.